1. Field of the Invention
The invention relates to a drive system and to a method for operating such a drive system, in particular for a motor vehicle.
2. Description of the Related Art
The invention can be applied to any vehicle, but the invention and the problems on which the invention is based will be explained in more detail with respect to a passenger car.
A hybrid vehicle generally is considered a vehicle with a drive system that uses a plurality of drive units, such as an internal combustion engine and an electric motor. Parallel hybrid drives usually should be used to generate the highest possible energetic efficiency. A parallel hybrid drive permits the electric motor and the internal combustion engine to apply torque to a transmission either alternatively or cumulatively. Furthermore, the electric motor can be as a generator. For example, brake energy is present in the form of kinetic energy when the vehicle is braked and can be recovered and used, for example, to charge electric energy stores. Frequent starting and acceleration that occur, for example, in road traffic, preferably are carried out or assisted by the electric motor in a hybrid motor vehicle because the operation of the internal combustion engine with frequent load changes results in increased fuel consumption and emissions of pollutants. In contrast to an internal combustion engine, an electric motor already has a high torque at low engine speeds, practically from the stationary state. As a result, an electric motor is suited particularly well for starting and acceleration processes. In contrast, an internal combustion engine can be operated with high efficiency only at its rated rotational speed, for example in the case of constant fast travel. To combine the advantages of an internal combustion engine with the advantages of an electric motor, it is therefore necessary to configure the drive system structurally in such a way that both the power of the internal combustion engine and the power of the electric motor can be input into the drive system.
U.S. Pat. No. 7,611,433 B2 describes a hybrid drive system for a motor vehicle having a double clutch transmission that has two component transmissions and one output shaft connected to the component transmissions via gearwheel stages. An electric machine arranged axially with respect to the output shaft can be connected to one end of the output shaft via an additional clutch. WO 2008/046185 A1 describes a further structural design of a hybrid drive system with an internal combustion engine and an electric motor. The electric motor is arranged axially with respect to an output shaft and can be connected to the output shaft via a clutch. These two designs require a large amount of axial space due to the arrangement of the electric motor. Furthermore, when the clutch assigned to the electric motor closes, the electric motor is connected directly to the drive axle of the vehicle via the output shaft. The drive axle rotates as soon as the electric motor is activated. It therefore is not possible to activate the electric motor in the stationary state of the vehicle, for example to start the internal combustion engine.
US 2003/0069103 A1 discloses a hybrid drive system for a motor vehicle with an automated conventional transmission, an internal combustion engine and an electric machine. The transmission has two component transmissions that optionally transmit the torque of the internal combustion engine to an output shaft. The electric machine is coupled to the output shaft of the transmission via a gear stage. This arrangement permits activation of the electric motor in the stationary state of the vehicle.
An object of the invention is to provide an improved drive system and an improved method for operating a drive system to overcome the above-mentioned disadvantages.